1. Field of the Invention
The present invention relates to a method and apparatus for controlling an automatic constant speed running apparatus which is used for automatically running an automotive vehicle at a preselected speed without any need to operate an accelerator pedal by automatically holding the traveling speed of the automotive vehicle at the preselected vehicle speed.
2. Description of the Prior Art
The Japanese Patent Examined Publication No. Hei 3-27412 shows an automatic constant speed running apparatus for automatically running a vehicle at a preselected vehicle speed.
In FIG. 4 which shows a control operation of an automatic constant speed running apparatus disclosed in the above-described publication, when the vehicle reaches an ascending sloped road during a constant speed control and a calculation result from a deviation of an actual vehicle speed relative to a memory vehicle speed and an acceleration obtained from a change rate of the actual vehicle speed per constant time is negative at a position a, a controller carries out a speed increasing control to an actuator in order to identify the actual vehicle speed with the memory vehicle speed. The speed increasing control is effected for a period of time obtained through the computation of the controller by using the deviation of the actual vehicle speed relative to the memory vehicle speed and the above-described acceleration. When the speed increasing control is effected to the actuator, a throttle valve connected to the actuator is opened to increase the speed of the vehicle so as to cause the actual vehicle speed to follow the memory vehicle speed.
When the predetermined computation result from the deviation of the actual vehicle speed relative to the memory vehicle speed and the acceleration obtained from the change rate of the actual vehicle speed per constant time becomes positive, the controller carries out a speed decreasing control to close the throttle valve to reduce the speed of the vehicle so as to cause the actual vehicle speed to follow the memory vehicle speed.
In the conventional automatic constant speed running apparatus, the predetermined computation result from the deviation of the actual vehicle speed relative to the memory vehicle speed and the acceleration obtained from the change rate of the actual vehicle speed per constant time becomes negative to thereby open the throttle valve. Accordingly, the opening degree of the throttle valve during the ascending sloped road running state is greater than that in the flat road running state. As a result, there is a fear that a shift-down control of an automatic transmission would be carried out from an overdrive (OD) mode to a drive (D3) mode at a position b in FIG. 4. If the shift-down control is effected by the automatic transmission, the actual vehicle speed is attenuated to the memory vehicle speed due to the increase of the engine torque. However, the engine torque is rapidly increased by the shift-down control under the condition that the throttle valve is kept opened. As a result, a so-called overshoot is generated and a plus deviation relative to the memory vehicle speed is generated at a position c. When the plus deviation is generated, the controller carries out the speed decreasing control to close the throttle valve. As a result of the closure of the throttle valve, the automatic transmission carries out the shift-up control from the drive (D3) mode to the overdrive (OD) mode in response to the opening degree of the throttle valve. As a result, in the case where the vehicle runs along a long ascending slope road, the foregoing control is repeatedly effected, resulting in degradation of running feeling. It is desired to overcome this problem.